Chapter 20

Chapter 20
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Blood Vessels and Circulation
General anatomy of blood vessels
Blood pressure, resistance and flow
Capillary exchange
Venous return and circulatory shock
Special circulatory routes
Anatomy of
– pulmonary circuit
– systemic arteries and veins
Anatomy of Blood Vessels
• Arteries carry blood away from heart
• Veins carry blood back to heart
• Capillaries connect smallest arteries to veins
Vessel Wall
• Tunica interna (intima)
– smooth inner layer that repels blood cells and platelets
– simple squamous endothelium overlying a basement
membrane and layer of fibrous tissue
• Tunica media
– middle layer
– usually thickest; smooth muscle, collagen, some
– smooth muscle for vasomotion
• Tunica externa (tunica adventitia)
– outermost layer
– loose connective tissue with vasa vasorum
Large Vessels
• Conducting (elastic) arteries - largest
– pulmonary, aorta and common carotid
– tunica media consists of perforated sheets of
elastic tissue, alternating with thin layers of
smooth muscle, collagen and elastic fibers
– expand during systole, recoil during diastole;
lessens fluctuations in BP
• Distributing (muscular) arteries
– distributes blood to specific organs; femoral
and splenic
– smooth muscle layers constitute 3/4 of wall
Medium Vessels
Arteries and Metarterioles
• Resistance (small) arteries
– arterioles control amount of blood to various
• Metarterioles
– short vessels connect arterioles to capillaries
– muscle cells form a precapillary sphincter
about entrance to capillary
Small Vessels
Arterial Sense Organs
• Major arteries above heart
• Carotid sinuses
– in walls of internal carotid artery
– monitors BP – signaling brainstem
• HR  and vessels dilate
• Carotid bodies
– oval bodies near carotids
– monitor blood chemistry
• adjust respiratory rate to stabilize pH, CO2, and O2
• Aortic bodies
– in walls of aorta
– same function as carotid bodies
• Thoroughfare channel - metarteriole
continues through capillary bed to venule
• Precapillary sphincters control which beds
are well perfused
– only 1/4 of the capillaries are open at a given
Control of Capillary Bed Perfusion
Control of Capillary Bed Perfusion
Types of Capillaries
• Continuous - occur in most tissues
– endothelial cells have tight junctions with intercellular
clefts (allow passage of solutes)
• Fenestrated - kidneys, small intestine
– organs that require rapid absorption or filtration
– filtration pores – spanned by very thin glycoprotein
layer - allows passage of only small molecules
• Sinusoids - liver, bone marrow, spleen
– irregular blood-filled spaces; some have extra large
fenestrations, allow proteins and blood cells to enter
Fenestrated Capillary
Sinusoid in Liver
• Veins
lower blood pressure: 10mmHg with little fluctuation
thinner walls, less muscular and elastic tissue
expand easily, have high capacitance
valves aid skeletal muscles in upward blood flow
• Venules
– postcapillary venules more porous than capillaries
– muscular venules have tunica media
• Venous sinuses
– veins with thin walls, large lumens, no smooth muscle
Blood Distribution, Resting Adult
Circulatory Routes
• Most common route
– heart  arteries 
arterioles  capillaries 
venules  veins
• Portal system
– blood flows through
two consecutive
capillary networks
before returning to heart
• hypothalamus - anterior
• found in kidneys
• between intestines - liver
• Point where 2 blood
vessels merge
• Arteriovenous shunt
– artery flows directly into
• Venous anastomosis
– most common, blockage
less serious
– alternate drainage of
• Arterial anastomosis
– collateral circulation
Principles of Blood Flow
• Blood flow: amount of blood flowing
through a tissue in a given time (ml/min)
• Perfusion: rate of blood flow per given
mass of tissue (ml/min/g)
• Important for delivery of nutrients and
oxygen, and removal of metabolic wastes
• Hemodynamics
– physical principles of blood flow based on
pressure and resistance
• F  P/R, (F = flow, P = difference in pressure,
R = resistance to flow)
Blood Pressure
Force that blood exerts against a vessel wall
Measured at brachial artery of arm
Systolic pressure: BP during ventricular systole
Diastolic pressure: BP during ventricular diastole
Normal value, young adult: 120/75 mm Hg
Pulse pressure: systolic - diastolic
– important measure of stress exerted on small arteries
• Mean arterial pressure (MAP):
– measurements taken at intervals of cardiac cycle, best
estimate: diastolic pressure + (1/3 of pulse pressure)
– varies with gravity: standing; 62 - head, 180 - ankle 20-22
BP Changes With Distance
Blood Pressure
• Importance of arterial elasticity
– expansion and recoil maintains steady flow of
blood throughout cardiac cycle, smoothes out
pressure fluctuations and  stress on small
• BP rises with age: arteries less distensible
• BP determined by cardiac output, blood
volume and peripheral resistance
Abnormalities of Blood Pressure
• Hypertension
– chronic resting BP > 140/90
– consequences
• can weaken small arteries and cause aneurysms
• Hypotension
– chronic low resting BP
– caused by blood loss, dehydration, anemia
Peripheral Resistance
• Blood viscosity - by RBC’s and albumin
–  viscosity with anemia, hypoproteinemia
–  viscosity with polycythemia , dehydration
• Vessel length
– pressure and flow  with distance (friction)
• Vessel radius - very powerful influence
over flow
– most adjustable variable, controls resistance
– vasomotion: change in vessel radius
• vasoconstriction, vasodilation
Peripheral Resistance
• Vessel radius (cont.)
– laminar flow - flows in layers, faster in center
– blood flow (F) proportional to the fourth power
of radius (r), F  r4
• arterioles can constrict to 1/3 of fully
relaxed radius
– if r = 3 mm, F = (34) = 81 mm/sec; if r = 1 mm, F =
– 3X  in radius results in 81X  in flow
Flow at Different Points
• From aorta to capillaries, flow  for 3
– greater distance, more friction to  flow
– smaller radii of arterioles and capillaries
– farther from heart, greater total cross
sectional area
• From capillaries to vena cava, flow  again
– large amount of blood forced into smaller
– never regains velocity of large arteries
Regulation of BP and Flow
• Local control
• Neural control
• Hormonal control
Local Control of BP and Flow
• Metabolic theory of autoregulation
– tissue inadequately perfused, wastes
accumulate = vasodilation
• Vasoactive chemicals
– substances that stimulate vasomotion;
histamine, bradykinin
• Reactive hyperemia
– blood supply cut off then restored
• Angiogenesis - growth of new vessels
– regrowth of uterine lining, around
obstructions, exercise, malignant tumors
– controlled by growth factors and inhibitors
Neural Control of BP and Flow
• Vasomotor center of medulla oblongata:
– sympathetic control stimulates most vessels
to constrict, but dilates vessels in skeletal and
cardiac muscle
– integrates three autonomic reflexes
• baroreflexes
• chemoreflexes
• medullary ischemic reflex
Neural Control: Baroreflex
• Changes in BP detected by stretch receptors
(baroreceptors), in large arteries above heart
– aortic arch
– aortic sinuses (behind aortic valve cusps)
– carotid sinus (base of each internal carotid artery)
• Autonomic negative feedback response
– baroreceptors send constant signals to brainstem
–  BP causes rate of signals to rise, inhibits vasomotor
center,  sympathetic tone, vasodilation causes BP 
–  BP causes rate of signals to drop, excites vasomotor
center,  sympathetic tone, vasoconstriction and BP 
Negative Feedback Response
Neural Control: Chemoreflex
• Chemoreceptors in aortic bodies and carotid
– located in aortic arch, subclavian arteries, external
carotid arteries
• Autonomic response to changes in blood
– pH, O2, CO2
– primary role: adjust respiration
– secondary role: vasomotion
• hypoxemia, hypercapnia and acidosis stimulate
chemoreceptors, instruct vasomotor center to cause
vasoconstriction,  BP,  lung perfusion and gas exchange
Other Inputs to Vasomotor Center
• Medullary ischemic reflex
– inadequate perfusion of brainstem
• cardiac and vasomotor centers send sympathetic
signals to heart and blood vessels
•  cardiac output and causes widespread
•  BP
• Other brain centers
– stress, anger, arousal can also  BP
Hormonal Control of BP and Flow
• Angiotensinogen (prohormone produced by liver)
 Renin (kidney enzyme released by low BP)
• Angiotensin I
 ACE (angiotensin-converting enzyme in lungs)
ACE inhibitors block this enzyme lowering BP
• Angiotensin II
– very potent vasoconstrictor
Hormonal Control of BP and Flow
• Aldosterone
– promotes Na+ and water retention by kidneys
– increases blood volume and pressure
• Atrial natriuretic factor ( urinary sodium excretion)
– generalized vasodilation
• ADH (water retention)
– pathologically high concentrations, vasoconstriction
• Epinephrine and norepinephrine effects
– most blood vessels
• binds to -adrenergic receptors, vasoconstriction
– skeletal and cardiac muscle blood vessels
• binds to -adrenergic receptors, vasodilation
Routing of Blood Flow
• Localized vasoconstriction
– pressure downstream drops, pressure
upstream rises
– enables routing blood to different organs as
• Arterioles - most control over peripheral
– located on proximal side of capillary beds
– most numerous
– more muscular by diameter
Blood Flow in Response to Needs
• Arterioles shift blood flow with changing priorities
Blood Flow Comparison
• During exercise
–  perfusion of lungs, myocardium and skeletal
muscles  perfusion of kidneys and digestive tract
Capillary Exchange
• Only occurs across capillary walls
between blood and surrounding tissues
• 3 routes across endothelial cells
– intercellular clefts
– fenestrations
– through cytoplasm
• Mechanisms involved
– diffusion, transcytosis, filtration and
Capillary Exchange - Diffusion
• Most important mechanism
• Lipid soluble substances
– steroid hormones, O2 and CO2 diffuse easily
• Insoluble substances
– glucose and electrolytes must pass through
channels, fenestrations or intercellular clefts
• Large particles - proteins, held back
Capillary Exchange - Transcytosis
• Pinocytosis - transport vesicles across cell exocytosis
• Important for fatty acids, albumin and some
hormones (insulin)
Capillary Exchange Filtration and Reabsorption
• Opposing forces
– blood (hydrostatic) pressure drives fluid out
of capillary
• high on arterial end of capillary, low on venous end
– colloid osmotic pressure (COP) draws fluid
into capillary
• results from plasma proteins (albumin)- more in
• oncotic pressure = net COP (blood COP - tissue COP)
• Hydrostatic pressure
– physical force exerted against a surface by a
liquid, (BP is an example)
Capillary Filtration and Reabsorption
• Capillary filtration at arterial end
• Capillary reabsorption at venous end
• Variations
– location
(glomeruli- devoted to filtration
alveolar cap.- devoted to absorption)
– activity or trauma
( filtration)
Causes of Edema
•  Capillary filtration ( capillary BP or permeability)
– poor venous return
• congestive heart failure - pulmonary edema
• insufficient muscular activity
– kidney failure (water retention, hypertension)
– histamine makes capillaries more permeable
•  Capillary reabsorption
– hypoproteinemia (oncotic pressure  blood albumin)
cirrhosis, famine, burns, kidney disease
• Obstructed lymphatic drainage
Consequences of Edema
• Tissue necrosis
– oxygen delivery and waste removal impaired
• Pulmonary edema
– suffocation
• Cerebral edema
– headaches, nausea, seizures and coma
• Circulatory shock
– excess fluid in tissue spaces causes low
blood volume and low BP
Mechanisms of Venous Return
• Pressure gradient
– 7-13 mm Hg venous pressure towards heart
• venules (12-18 mm Hg) to central venous pressure (~5 mm Hg)
• Gravity drains blood from head and neck
• Skeletal muscle pump in the limbs
• Thoracic pump
– inhalation - thoracic cavity expands (pressure )
abdominal pressure , forcing blood upward
– central venous pressure fluctuates
• 2mmHg- inhalation, 6mmHg-exhalation
• blood flows faster with inhalation
• Cardiac suction of expanding atrial space
Skeletal Muscle Pump
Venous Return and Physical Activity
• Exercise  venous return in many ways
– heart beats faster, harder -  CO and BP
– vessels of skeletal muscles, lungs and heart dilate 
–  respiratory rate  action of thoracic pump
–  skeletal muscle pump
• Venous pooling occurs with inactivity
– venous pressure not enough force blood upward
– with prolonged standing, CO may be low enough to
cause dizziness or syncope
• prevented by tensing leg muscles, activate skeletal m. pump
– jet pilots wear pressure suits
Circulatory Shock
Any state where cardiac output
insufficient to meet metabolic needs
– cardiogenic shock - inadequate pumping of
heart (MI)
– low venous return (LVR) shock - 3 principle
1. hypovolemic shock - most common
– loss of blood volume: trauma, burns, dehydration
2. obstructed venous return shock
– tumor or aneurysm
3. venous pooling (vascular) shock
– next slide
LVR Shock
• Venous pooling (vascular) shock
– long periods of standing, sitting or widespread
– neurogenic shock - loss of vasomotor tone,
• causes from emotional shock to brainstem injury
• Septic shock
– bacterial toxins trigger vasodilation and  capillary
• Anaphylactic shock
– severe immune reaction to antigen, histamine release,
generalized vasodilation,  capillary permeability
Responses to Circulatory Shock
• Compensated shock
• Decompensated shock
Compensated shock
• Homeostatic mechanisms bring about
•  BP triggers baroreflex and production of
angiotensin II, both stimulate
• If person faints and falls to horizontal
position, gravity restores blood flow to
brain; quicker if feet are raised
Decompensated shock
• Life threatening positive feedback loops occur
–  CO  myocardial ischemia and infarction 
 CO
– slow circulation  disseminated intravascular
coagulation  slow circulation
– ischemia and acidosis of brainstem  
vasomotor tone, vasodilation   CO 
ischemia and acidosis of brainstem
Special Circulatory Routes- Brain
• Total perfusion kept constant
– seconds of deprivation causes loss of consciousness
– 4-5 minutes causes irreversible brain damage
– flow can be shifted from one active region to another
• Responds to changes in BP and chemistry
– cerebral arteries: dilate as BP , constrict as BP rises
– main chemical stimulus: pH
• CO2 + H2O  H2 CO3  H+ + (HCO3)• hypercapnia (CO2 ) in brain, pH , triggers vasodilation
• hypocapnia,  pH, vasoconstriction
– occurs with hyperventilation, may lead to ischemia,
dizziness and sometimes syncope
TIA’s and CVA’s
• TIA’s - transient ischemic attacks
– dizziness, loss of vision, weakness, paralysis,
headache or aphasia; lasts from a moment to a few
hours, often early warning of impending stroke
• CVA - cerebral vascular accident (stroke)
– brain infarction caused by ischemia
• atherosclerosis, thrombosis, ruptured aneurysm
– effects range from unnoticeable to fatal
• blindness, paralysis, loss of sensation, loss of speech
– recovery depends on surrounding neurons, collateral
Special Circulatory Routes Skeletal Muscle
• Highly variable flow
• At rest
– arterioles constrict, total flow about 1L/min
• During exercise
– arterioles dilate in response to epinephrine and
sympathetic nerves
– precapillary sphincters dilate due to lactic acid, CO2
– blood flow can increase 20 fold
• Muscular contraction impedes flow
– isometric contraction causes fatigue faster than
Special Circulatory Routes Lungs
• Low pulmonary blood pressure
– flow slower, more time for gas exchange
– capillary fluid absorption
• oncotic pressure overrides hydrostatic pressure
• Unique response to hypoxia
– pulmonary arteries constrict, redirects flow to
better ventilated region
Pulmonary Circulation
• Pulmonary trunk to pulmonary arteries to lungs
– lobar branches for each lobe (3 right, 2 left)
• Pulmonary veins return to left atrium
– increased O2 and reduced CO2 levels
Pulmonary Capillaries Near Alveoli
• Basketlike
capillary beds
• Exchange of
gases with air
at alveoli
Major Systemic Arteries
• Supplies oxygen and nutrients to all organs
Major Branches of Aorta
• Ascending aorta
– right and left coronary arteries supply heart
• Aortic arch
– brachiocephalic
• right common carotid supplying right side of head
• right subclavian supplying right shoulder and upper limb
– left common carotid supplying left side of head
– left subclavian supplying shoulder and upper limb
• Descending aorta
– thoracic aorta above diaphragm
– abdominal aorta below diaphragm
Major Branches of the Aorta
Arteries of the Head and Neck
• Common carotid to internal and external carotids
– external carotid supplies most external head structures
Arterial Supply of Brain
• Paired vertebral aa. combine to form basilar artery on pons
• Circle of Willis on base of brain formed from anastomosis of
basilar and internal carotid aa
• Supplies brain, internal ear and orbital structures
– anterior, middle and posterior cerebral
– superior, anterior and posterior cerebellar
Arteries of the Upper Limb
• Subclavian
passes between
clavicle and 1st
• Vessel changes
names as passes
to different
– subclavian to
axillary to brachial
to radial and ulnar
– brachial used for
BP and radial
artery for pulse 20-67
Arteries of the Thorax
• Thoracic aorta supplies viscera and body wall
– bronchial, esophageal and mediastinal branches
– posterior intercostal and phrenic arteries
• Internal thoracic, anterior intercostal and
pericardiophrenic arise from subclavian artery
Major Branches of Abdominal Aorta
Celiac Trunk Branches
• Branches of celiac trunk supply upper
abdominal viscera -- stomach, spleen, liver and
Mesenteric Arteries
Arteries of the Lower Limb
• Branches to the lower limb arise from external
iliac branch of the common iliac artery
Arterial Pressure Points
• Some major arteries close to surface -- allows
palpation for pulse and serve as pressure
points to reduce arterial bleeding
Major Systemic Veins
• Deep veins run parallel to arteries while
superficial veins have many anastomoses
Deep Veins of Head and Neck
• Large, thin-walled dural sinuses form in between
layers of dura mater (drain brain to internal
jugular vein)
Superficial Veins of Head and Neck
• Branches of internal and external jugular veins
drain the external structures of the head
• Upper limb is drained by subclavian vein
Superficial and Deep Veins of Upper
Inferior Vena Cava and Branches
• Notice absence of veins draining the viscera --20-78
stomach, spleen, pancreas and intestines
Veins of Hepatic Portal System
• Drains blood from viscera (stomach, spleen and
intestines) to liver so that nutrients are absorbed
Superficial and Deep Veins of Lower Limb
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